Is render time, openGL, or memory affected whether a modifier is applied or not?
Viewport speed: The final displayed mesh is the same regardless of if you apply the modifier or not. Applying it will cost you the ability to have different settings for viewport and render for modifiers that do that, such as on the subsurf modifier.

Scene update (opening file, changing frames, etc): Applying bakes in the modifier, so it doesn't need to be calculated on updates anymore. So that's faster.

Render: modifiers "apply" on the fly when exporting meshes for render, so no  difference. Same output mesh is rendered either way. The modifiers have to all be updated during this process though, so applying will speed render startup a bit. 

So generally, not much performance reason to apply a modifier. The two main situations where you'd apply it are when you need to do something on the modifier output directly (like applying mirror modifier before rigging), or when a modifier is taking a long time to calculate and losing non-destructive editing of the modifier is preferable to waiting that out.